1. Field of the Invention
The present invention relates to a fuel cell separator used in combination with an electrolyte electrode assembly to form a unit cell of a fuel cell and a method of producing the fuel cell separator.
2. Description of the Related Art
In general, a fuel cell includes unit cells each formed by sandwiching an electrolyte electrode assembly between a pair of separators. In the unit cell having this structure, seals are formed at edges on both surfaces of the separators (e.g., see Japanese Laid-Open Patent Publication No. 2004-207071). In operation of the fuel cell, a fuel gas containing hydrogen is supplied to the anode of the electrolyte electrode assembly, and an oxygen-containing gas is supplied to the cathode of the electrolyte electrode assembly. The seals are used to prevent leakages of the fuel gas and the oxygen-containing gas to the outside of the fuel cell.
Further, as described in Japanese Laid-Open Patent Publication No. 2005-222764, the seals may be provided around a coolant flow field as a passage of a coolant. Further, the seals may be provided around an oxygen-containing gas flow field as a passage of the humidified oxygen-containing gas and a fuel gas flow field as a passage of the humidified fuel gas. The seals are provided because condensation may occur in the oxygen-containing gas or the fuel gas, and water produced in the power generation operation of the fuel cell may be retained in the oxygen-containing gas flow field or the fuel gas flow field. It is a matter of course that the seals may be provided for all of the coolant flow field, the oxygen-containing gas flow field, and the fuel gas flow field.
As the seals (seal composition) of this type, silicone resin is adopted widely. The silicone resin has good elasticity, and easily absorbs expansion/contraction of the stack during operation of the fuel cell, or when operation of the fuel cell is stopped. Further, since the elasticity of the silicone resin is maintained even at the temperature below the freezing point, it is possible to prevent leakage of the reactant gases even in a cold region or the like. Therefore, the silicone resin can be used suitably for fuel cells in automobile applications.
However, the acid resistance of the silicone resin may not be sufficient in some applications. In general, the electrolyte membrane of the electrolyte electrode assembly has high acidity. Therefore, the silicone resin adjacent to the electrolyte membrane may be degraded, and the elasticity may be lowered undesirably. Likewise, since the primer provided between the silicone resin and the separator to adhere the silicone resin to the separator is degraded by the acid, the seal may be peeled off from the separator undesirably.
In this regard, in a proposed technique disclosed in Japanese Laid-Open Patent Publication No. 2002-083616, cross-linking reaction between predetermined liquids is induced to obtain a fuel cell packing material (seal) which is made of addition type silicone having good acid resistance. Further, in a proposed technique disclosed in Japanese Laid-Open Patent Publication No. 2006-206616, acid resistance is improved using resol-type phenolic resin and primer compound containing organic compound having a chelate ring and/or an alkoxyl group.
In operating the fuel cell, the temperature of the fuel cell is raised to a predetermined operating temperature. Then, as known in the art, by operation of the fuel cell, H2O (chiefly water vapor) is produced. This H2O is discharged from the flow field together with the fuel gas consumed at the anode or the oxygen-containing gas consumed at the cathode.
As can be seen from the above, hot and highly humid gases contact the fuel cell seal. Though the silicone rubber has sufficient gas sealing performance for preventing leakages of the fuel gas and the oxygen-containing gas, the gas permeability of the silicone rubber is large in comparison with the Fluoro Rubber or the EPDM (ethylene propylene diene monomer) rubber. Since the silicone rubber has water repellency, the water permeability of the silicone rubber is extremely small. Therefore, in the case where the primer is made of the silicone rubber, the water vapor (gas) passes through the silicone rubber, and the water vapor is condensed into the liquid state at the interface between the silicone rubber and the separator. In this case, blisters may be formed undesirably. If the blisters are formed near the flow field, the sectional area of the flow field is reduced, and pressure losses occur undesirably.
Thus, it is desired to sufficiently prevent formation of blisters in the fuel cell seal. However, in the conventional techniques as noted above, it is difficult to reliably prevent formation of blisters.